Transparent films are being increasingly used as coatings for various glass and plastic articles, such as glass or plastic optical lenses; goggles; face shields; face plates for helmets; automobile windshields; substrates used between temperature differentials, i.e., frozen food display doors; and the like. Organic polymer coatings, such as polyurethanes, are particularly useful in providing self-supporting surfaces or coatings for such applications, in that these transparent materials can provide high strength and improved abrasion resistance to the surface. However, such polyurethane coatings have a high susceptibility to fogging (also referred to as frosting under certain conditions) on their surfaces, which wiping of the surface cannot always adequately remedy.
Fog appears when moisture condenses on a hydrophobic surface and is drawn into tiny droplets that scatter light. The scattering of the light gives the surface the appearance of a fog. Hydrophilic surfaces, on the other hand, will absorb the condensed moisture into the surface preventing the tiny light scattering droplets from forming. However, at some point the hydrophilic surface may reach saturation of the moisture, thus resulting in the formation of light scattering water droplets on the surface and resulting in poor anti-fog coatings. Hydrophilic surfaces which absorb the condensed moisture may also swell in an undesirable manner. Hydrophilic and hydrophobic surfaces can be modified by surface active agents, which are also known as surfactants, to spread or sheet the water across the surface. Surfactants containing both hydrophilic and hydrophobic segments act to minimize surface tension of the water with respect to the surface in an effect called “wetting.” The “wetting” of the surface spreads or sheets the water out across the surface, thus minimizing the light scattering effect of the water droplets.
Surfactants are used to provide anti-fog properties to both hydrophilic and hydrophobic surfaces, including polyurethane polymer surfaces formed from polyurethane coatings. These surfactants can be physically associated with the polyurethane surface, such as being applied externally as a temporary film by wiping or spraying the surface. Alternatively, the surfactants can be mixed in with the polyurethane coating composition before it is applied to a substrate, so that the surfactant becomes physically trapped within the polyurethane polymer structure as the polyurethane cures. Such surfactants that are only physically associated with the polyurethane coating are easily washed off or leached away, thereby resulting in temporary anti-fog properties for the polyurethane surface.